This application is based on application Ser. No. 2000-010627 filed on Jan. 19, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an electric power steering apparatus for a vehicle capable of lightening the steering operation of an operator at the time of deceleration of the vehicle, and improving a deterioration in the steerability or maneuverability of a steering wheel caused by irregularities on road surfaces or the like during such a time, as well as improving a steering feeling or sensation of the operator.
2. Description of the Related Art
Conventionally, a variety of vehicle speed sensitive type electric power steering apparatuses have been proposed in which the heaviness in operation of a steering wheel by the operator during steering operation can be varied in response to vehicle speeds.
FIG. 13 is a schematic block diagram showing an arrangement of a conventional speed sensitive type electric power steering apparatus as disclosed in, for example, Japanese Patent Publication No. Hei 8-15865. FIG. 14 is a characteristic diagram for explaining the amount of power assist used in the conventional electric power steering apparatus. FIG. 15 is a conceptual diagram showing a steering mechanism employed in the conventional electric power steering apparatus. FIG. 16 illustrates an external appearance of a steering gear unit in an enlarged manner, employed in the conventional electric power steering apparatus.
In FIG. 13, the electric power steering apparatus includes a vehicle speed sensor 1 for sensing the driving or travelling speed of a vehicle, a torque sensor 2 for sensing steering torque applied to a steering wheel 9 (see FIG. 15) of a vehicle on which the apparatus is mounted, and a controller 3 for determining the amount of power assist given to the steering wheel by an actuator 5 in the form of an electric motor based upon the driving speed sensed by the vehicle speed sensor 1 and the steering torque sensed by the torque sensor 2. A driver 4 receives the power assist force as a control signal from the controller 3 and serves to drive/control the actuator 5 based thereon. A steering gear 6 is engaged with the output shaft of the actuator 5 for steering driven wheels of the vehicle.
As shown in FIG. 15, the actuator 5a is provided at its output shaft with a speed reducer 5a so that the output power of the actuator reduced by the speed reducer 5a is transmitted to a rack 6a which is meshed with the steering gear 6. A gear case 8 encloses the steering gear 6 and the rack 6a meshed with the steering gear 6. A steering force or torque applied to the steering wheel 9 by the vehicle operator is transmitted through the steering gear 6 to driven wheels 10.
As depicted in FIG. 14, in the conventional power steering apparatus employing the above arrangement, the rate of increase in the power assist force is variable so that it increases in accordance with the increasing steering torque when the vehicle speed is low, whereas it decreases in accordance with the increasing steering torque when the vehicle speed is high. As a result, in the high speed range, the power assist force is reduced to decrease the steering amount, so that the heaviness in operation of the steering wheel can be increased.
In the conventional vehicle speed sensitive type electric power steering apparatus, when the vehicle is rapidly decelerated, a greater portion of the vehicle load or weight is applied to the front wheels, thus giving rise to the following problem. That is, the electric power steering apparatus is subject to an adverse influence due to irregularities on the road surfaces so that the steering wheel becomes liable to be forced to move unintentionally. As a result, the steering characteristic of the vehicle is deteriorated.
The present invention is intended to obviate the above-described problem, and therefore, has an object to provide an electric power steering apparatus for a vehicle which is capable of lightening the heaviness in operation of the steering wheel in response to a deceleration of the vehicle, and of improving the steering characteristic and the steering feeling or sensation of the operator when a vehicle speed is rapidly reduced.
Bearing the above object in mind, according to the present invention, there is provided an electric power steering apparatus for a vehicle for applying an assist force to a steering system so as to increase an operator""s induced steering force, the apparatus comprising a vehicle speed detecting unit for detecting a speed of the vehicle, a torque detecting unit for detecting steering torque, a deceleration calculating unit for calculating a deceleration of the vehicle from the vehicle speed detected by the vehicle speed detecting unit, a first assist force calculating unit for calculating a first assist force from the detection results of the vehicle speed detecting unit and the torque detecting unit, a second assist force calculating unit for calculating a second assist force in response to the deceleration calculated by the deceleration calculating unit, and a control unit for applying to the steering system an assist force which is a sum of the first assist force calculated by the first assist force calculating unit and the second assist force calculated by the second assist force calculating unit.
In a preferred form of the invention, the electric power steering apparatus further comprises a brake detecting unit for detecting a braking operation by an operator of the vehicle, and when the brake detecting unit detects the braking operation by the operator, the deceleration calculating unit calculates the deceleration of the vehicle.
In another preferred form of the invention, the electric power steering apparatus further comprises a vehicle load detecting unit for detecting a vehicle load applied to a front wheel of the vehicle, and a third assist force calculating unit for calculating a third assist force in response to the vehicle load. The control unit applies to the steering system an assist force which is a total sum of the third assist force calculated by the third assist force, the first assist force and the second assist force.
In a further preferred form of the invention, the electric power steering apparatus further comprises a deceleration threshold value calculating unit for calculating a deceleration threshold value in response to a vehicle speed, and a second assist force calculation judging unit for judging as to whether or not the second assist force is to be added to the first assist force, based upon a comparison result between the deceleration calculated by the deceleration calculating unit and the deceleration threshold value calculated by the deceleration threshold value calculating unit.
In a still further preferred form of the invention, the electric power steering apparatus further comprises a vehicle load threshold value calculating unit for calculating a vehicle load threshold value in response to a vehicle speed, and a third assist force calculation judging unit for judging as to whether or not the third assist force is to be added to the sum of the first assist force and the second assist force based upon a comparison result between the vehicle load detected by the vehicle load detecting unit and the vehicle load threshold value calculated by the vehicle load threshold value calculating unit.